Polyoxymethylene dialkyl ethers (RO(CH2O)nR) are novel blending components for clean oils, which has very high cetane number (H3CO(CH2O)2CH3: 63, H3CO(CH2O)3CH3: 78, H3CO(CH2O)4CH3: 90, H3CO(CH2O)5CH3: 100) and oxygen contents (methyl series: 42%-49%, ethyl series: 30%-43%). When polyoxymethylene dialkyl ethers are added into diesel oil in an amount of 10%-20%, they can improve the burning characteristic of diesel oil remarkably, increase the thermal efficiency effectively, and reduce the discharge of NOx and carbon smoke greatly, and therefore are regarded as blending components for the environmentally acceptable diesel oils with a promising application prospect. It was reported that when 5%-30% of H3COCH2OCH3 was added into diesel oil, the NO content in tail gas could be reduced by 7%-10%, and the particulate pollutants could be reduced by 5%-35%.
In the earlier time, polyoxymethylene dimethyl ethers (DMMn) are synthesized by taking methanol, formaldehyde, paraformaldehyde, or glycol ethylidene-formal as starting material under the catalysis of sulfuric acid or hydrochloric acid. The reaction should be carried out at a relatively high temperature and will produce a great amount of CO2 as byproduct. In 1948, Du Pont (U.S. Pat. No. 2,449,469) investigated the acetalation reaction of polyoxymethylene ether with formaldehyde or paraformaldehyde using an inorganic acidic catalyst such as sulfuric acid under a relatively mild condition, which mainly produced a polyoxymethylene dialkyl ethers wherein n=2-3.
In 2001, Snamprogetti S.P.A. (EP 1070755 A1) reported an acetalation reaction of paraformaldehyde with methylal catalyzed by an inorganic acid such as trifluoromethane sulfonic acid, which was reacted at 115° C./2.0 MPa for 40 min, giving a main product of DMM2-5 in a yield of up to 51.2%. Subsequently, the process of the reaction was investigated (EP 1505049 A1). The reaction liquid containing the catalyst was adsorbed on a silica gel column to remove the acid and water. The treated reaction liquid was introduced into a rectification column and separated into light components (trioxymethylene and DMM1-2), target products (DMM3-5) and heavy components (DMM≧5) by a two-stage rectification method, wherein the light components and the heavy components were recycled to the reactor for repeated use.
BASF (WO 2006/045506 A1, CA 2581502) reported an acetalation reaction using trioxymethylene as the source of formaldehyde and being catalyzed by protonic acid such as sulfuric acid, trifluoromethane sulfonic acid and acidic cation exchange resin, wherein the reaction was performed at 100° C. for 8-12 h to obtain a series of products wherein n=2-10. The catalyst-containing reaction liquid was adsorbed on a packing column packed with an anion exchange resin to remove the acid and water. The treated reaction liquid was introduced into a rectification column to separate the products by three-stage rectification, wherein the selectivity for DMM3-4 is not above 25.7%. The DMMn with n≦2 and n≧5 was recycled to the reactor for repeated use.
In the above processes of acetalation reaction, the liquid acid catalysts and the products are both in liquid phase, and the catalysts are separated from the reaction liquids by a manner of adsorption or the like. This separation process is complex, and the catalysts cannot be recycled. Additionally, the energy consumption of processing is high. The distribution of products is not desirable and the yield of the component DMM3-8 which can be used as an oil additive is not high.
BP Company developed heterogeneous catalyst systems of borosilicate molecular sieve, sulfonic acid-based cation exchange resin or the like (U.S. Pat. Nos. 5,959,156, 6,160,174, 62,655,284). Dimethyl ether and methanol were used as the starting reaction materials to produce formaldehyde via a hydration reaction of dimethyl ether. Further, an acetalation reaction of formaldehyde and methanol produced DMMn. In this reaction process, the separation of the products (DMM≧2) and the reuse of the raw materials were realized in a reactive rectification manner. However, the catalyst in this method had a low activity and had to be regenerated frequently, leading to a complex process.
In recent years, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (U.S. Pat. No. 7,560,599 B2) reported a method for synthesizing DMMn by an acetalation reaction of trioxymethylene and methanol catalyzed by an ionic liquid, wherein the reaction conversion could be up to 90% and the selectivity for DMM3-8 could be up to 40%. The separation and recycling of the catalyst were realized (CN 200810150868.4).